what are virtual twins

Virtual twins are two children who are generally the same age and who are not genetic siblings, yet they are "reared together since infancy". Another definition states that virtual twins are "unrelated children born within nine months of each other who enter a family, through birth or adoption, in the first year of life". This phenomenon has led to the creation of the Fullerton Virtual Twin Study (FVTS) at California State University, Fullerton. Virtual twins are sometimes referred to as "almost twins," "twins of sorts," or "twiblings". They are not biologically related, but their ages are within 9 months of each other, and they become siblings through adoption or birth. Virtual twins replicate the rearing situation of twins, but without the genetic link. The concept of virtual twins has been the subject of research into behavioral differences among twins.